n-api: mark thread-safe function as stable

Fixes: https://github.com/nodejs/node/issues/24249
PR-URL: https://github.com/nodejs/node/pull/25556


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>